SIMS analysis of the oxidation behaviour of SiFeCr coated technical refractory metal alloys

Refractory metal alloys play an important role as highly thermostable materials in the air- and spacecraft industry. Due to their lack of oxidation stability at elevated temperatures it is necessary to protect them with coatings. The thermal-cycle-stability of Si20Cr20Fe-coatings on Nb10W2.5Zr and Ta10W was investigated by means of imaging Secondary Ion Mass Spectrometry; this showed the inward-diffusion of oxygen and other contaminants along cracks formed in the coating during thermal cycling and subsequent precipitation of oxides at the grain boundaries, leading to brittle fracture behaviour of the oxygen-affected surface zone. EDX measurements have been taken to determine the composition of the phases established in this complex system.